1. Field of the Invention
The present invention relates to a method for producing a large colorless high purity single crystal diamond using Microwave Plasma Chemical Vapor Deposition (MPCVD) without using a subsequent annealing step.
2. Description of Related Art
Large-scale production of synthetic diamond has long been an objective of both research and industry. Diamond, in addition to its gem properties, is the hardest known material, has the highest known thermal conductivity, and is transparent to a wide variety of electromagnetic radiation. These and other characteristics, therefore, make diamond very valuable industrially and open up a wide range of applications in a number of industries, in addition to its well-established value as a gemstone.
For at least the last twenty years, a process of producing small quantities of diamond by chemical vapor deposition (CVD) has been available. See B. V. Spitsyn et al., “Vapor Growth of Diamond on Diamond and Other Surfaces,” Journal of Crystal Growth, Vol. 52, pp. 219-226. The process involves chemical vapor deposition of diamond on a substrate by using a combination of methane, or another simple hydrocarbon gas, and hydrogen gas at reduced pressures and temperatures of 800-1200° C. Hydrogen gas is included to prevent the formation of graphite as the diamond nucleates and grows. Growth rates of up to 1 μm/hour were reported with this technique.
Subsequent work, for example, that of Kamo, et al. as reported in “Diamond Synthesis from Gas Phase in Microwave Plasma,” Journal of Crystal Growth, vol. 62, pp. 642-644, has demonstrated the use of MPCVD to produce diamond at pressures of 1-8 kPa and temperatures of 800-1000° C. with microwave power of 300-700 W at a frequency of 2.45 GHz. A concentration of 1-3% methane gas was used in the process of Kamo et al. Maximum growth rates of 3 μm/hour were reported using this MPCVD process. In the above-described processes, and in a number of other reported processes, the growth rates are limited to only a few micrometers per hour.
Until recently, the known higher-growth rate processes only produced polycrystalline forms of diamonds. However, single crystal diamonds offer a variety of advantages over polycrystalline diamonds. Accordingly, considerable interest has been shown in recent years towards developing procedures which enable the fast growth of single-crystal CVD diamond by MPCVD.1-4 It has been reported, for example, that nitrogen addition to MPCVD reaction chemistry (methane/hydrogen plasma) can significantly enhance the growth of {100} facets and produce smooth and continuous diamond surfaces.1,5 Yan et al.1 originally reported high growth rates of up to 100 μm/hr, two orders of magnitude higher than standard processes for making CVD diamond at the time. Since then, efforts have been made to increase the growth rate3 or expand the growth area4 for single-crystal CVD diamond.
Considerable interest has also been shown in recent years towards developing procedures to produce clear, colorless, high optical quality single crystal diamond by chemical vapor deposition. Typically, such procedures have included the additional step of annealing the diamond produced by chemical vapor deposition. A need exists, however, to efficiently produce clear, colorless, high optical quality single crystal diamond using only the chemical vapor deposition process.